A couple in Florida is desperately trying to find a lost $350,000 10-carat ring that disappeared from their house on Dec. 23. The next day, another Florida man reunited a 75-year-old man with his grandmother's ring that had been lost since the 1930s.

The missing ring was last seen on Dec. 23 when the owner put it in a cup on top of her dresser in her home in Martin County, Fla. When it disappeared, the couple hired leading Florida private investigator John Milano to search for the bauble.

"We're trying every route we can possibly think of. She is distraught beyond compare," Milano told ABCNews.com. "I don't think anybody who has it knows the value of it, if they do have it."

Milano has performed polygraph tests on the staff who works in the house. Two were cleared right away and another is pending. The couple and Milano believe the ring was stolen and not misplaced.

"We are pretty convinced that it was removed. It wasn't lost," he said.

The sparkler is very unusual, and Milano said it would be extremely difficult to sell and go undetected.

"The main stone is an oddity, a 10.38-carat emerald cut diamond," Milano said. "The side diamonds are also one carat each and kite-shaped, which is an unusual cut."

Beyond the house staff, Milano has looked into local pawn stores, places that purchase gold and dealers in New York, a diamond market hub. Though he has extensive investigative experience, Milano said this ring was a first.

"I've never looked for anything like this at all. Lots of cheating spouses ... we do mostly criminal defense, a few missing cars and motorcycles, but other than that, I've never looked for a $350,000 ring, that's for sure," he said, laughing.

The couple has not filed a police report because they wish to remain anonymous for privacy and security reasons, Milano said. But they hope that media exposure of the ring will lead to its return. They are also offering a $5,000 reward for information leading to the ring.

As Milano searches fervently for the ring, a Florida man who was not even looking for a ring found a family treasure that had been missing for at least 80 years.

Reed Banjanin, 38, is an electronics technician for AT&T in the Orlando area, and an avid diver. He lives in the Springs, a gated subdivision with a private spring. From 1930 to 1965, however, the spring was open to the public.

After diving in the spring several times a week for months, Banjanin grew bored of the location and stopped until July 2012, when "something compelled me to go down" with his underwater metal detector, he told ABCNews.com.

"I went down there and right at the mouth of the cave where everything settles, in a crevice, I got a hit," Banjanin said. "I dug down and there was a gold ring sitting there in perfect condition."

The gold ring was a class ring from the Women's College of Mississippi. The year 1923 was on it, and the name Louise Hearst was engraved in a pretty font on the inside.

"I just felt compelled to find the owner of this ring," Banjanin said.

He found out right away that the Women's College of Mississippi had become the William Carey University, located in Hattiesberg, Miss. He found Hearst in an online class record, but all it had was her name and graduation date.

Banjanin spent about a month looking for clues about a Louise Hearst in Hattiesberg but continually hit dead ends and was unsure what to do next.

He posted a photograph of his ring and his story on Facebook and got a few responses, but no real leads. A friend encouraged him to keep looking, and he picked up the search again. He eventually found a marriage record that revealed that Hearst had married and become Louise Hearst Entzminger.

Census records showed she had lived near the Springs in the 1930s when it was public property and known as Sanlando Springs. Banjanin believes Hearst likely lost the ring while swimming in the spring.

With that information, he could uncover that she'd died and was buried in Bylthewood, S.C. He tracked down her grandson, John Entzminger, in Oakton, Va., and contacted him in December 2011.

"It was [Entzminger's] birthday, and I called him," Banjanin said. "He was just amazed. He's also a Christian, and we both think it was meant to be right at that time."

"I said, 'Wow I can't believe it!'" Entzminger told ABCNews.com of his reaction when Banjanin called him. "I got it on Christmas Eve. It really made a great present."

Though Banjanin has been on countless dives, he said this one was unique.